The present invention relates to a tetrafluoroethylene copolymer. In particular, the present invention relates to a tetrafluoroethylene copolymer which has good transparency and can be adhered to substrates made of various organic and inorganic materials, and a laminate, a film and a surface film or a transparent filler of a solar cell comprising the same.
Conventionally used fluororesins for molding include tetrafluoroethylene-hexafluoropropylene copolymers (FEP), tetrafluoroethylene-perfluoro(alkyl vinyl ether) copolymers (PFA), tetrafluoroethylene-ethylene copolymers (ETFE), and the like.
Such fluororesins are widely used in various fields such as automobiles, industrial machines, office automation equipment, electric or electronic devices, etc. since they have good heat resistance, chemical resistance, solvent resistance, weather resistance, sliding properties, electrical insulation properties, flame retardance, etc.
However, fluororesins has drawbacks such that they have insufficient mechanical strength or dimensional stability, or they are expensive.
Thus, it has been investigated to adhere or laminate a fluororesin to other organic or inorganic materials in order to make the best use of its advantages but minimize its disadvantages.
However, since fluororesins have small surface energies, they have poor affinity with other materials, and small adhesion force. Thus, it is difficult to directly adhere fluororesins to other materials or substrates. When fluororesins are heat bonded to other materials, the bond strength is still insufficient.
To adhere fluororesins to other materials, the following methods are investigated:
1. The surface of a fluororesin film is treated by sodium etching, plasma treatment, or photochemical treatment, and then the film is adhered to a substrate.
2. The surface of a substrate is physically roughened by sand blasting, etc., and then the substrate is adhered to a fluororesin material.
3. A fluororesin material and a substrate are adhered with an adhesive.
The above methods 1 and 2 require the pretreatment, and the whole steps become complicated, and thus they have low productivity. In addition, the methods 1 and 2 achieve insufficient bond strength, and cause some problems in the appearance of a laminate, such as coloring or easy damaging.
Various adhesives have been inventigated for use in the above method 3. In general, adhesives comprising hydrocarbons have insufficient adhesion properties with fluororesins, and thus the adhesive layers of laminates have insufficient chemical resistance, water resistance and weather resistance. Accordingly, the laminates lose the adhesion strength due to the change of temperature or environmental conditions, and thus have low reliability.
Conventional fluororesins should be molded at a temperature of 300xc2x0 C. or higher, since they have a melting point of 250xc2x0 C. or higher. Such a molding temperature is higher than the appropriate molding temperatures of general-purpose resins (e.g. polyamide, polyester, polymethacrylate, polyvinyl chloride, polystyrene, ABS, polyacetal, polycarbonate, epoxy resin, polyurethane, etc.) and fiber-reinforced plastics (FRP) comprising such general-purpose resins.
Thus, a temperature of not lower than 300xc2x0 C. is necessary to heat bond the films of such general-purpose resins and the films of the fluororesins. However, the general-purpose resins are decomposed at such a high temperature to cause foaming or coloring, which are practically unpreferable. It is difficult to co-extrude general-purpose resins and fluororesins to form a multilayer laminate, since they have different molding temperatures as explained above.
It is known for a long time to copolymerize tetrafluoroethylene with various comonomers to modify the properties of polytetrafluoroethylene.
For example, JP-A-49-98488 discloses a terpolymer comprising 20 to 30 mole % of tetrafluoroethylene (TFE), 40 to 60 mole % of ethylene (Et) and 10 to 30 mole % of hexafluoropropylene (HFP). This patent publication describes that the advantageous properties of this terpolymer can be attained only in the specified composition range. A terpolymer prepared in an Example contains 46 to 50 mole % of Et. Thus, the content of TFE is relatively low, and thus the terpolymer has low weather resistance, chemical resistance, flame retardance, non-tackiness and stain resistance which are the inherent properties of fluororesins.
U.S. Pat. No. 4,338,237 discloses a method for the preparation of a stabilized aqueous colloidal dispersion containing a TFE-Et-HFP terpolymer. The monomeric composition of the terpolymer is 30 to 60 mole % of TFE, 40 to 60 mole % of Et and 0 to 15 mole % of HFP. A terpolymer prepared in an Example contains 4.5 or 4.7 mole % of HFP, and 46.5 or 46.8 mole % of Et. However, the specification of this U.S. Patent includes no description indicating a polymer composition which improves the adhesion properties of a terpolymer to various substrates.
JP-A-8-41131 discloses a terpolymer comprising 45 to 55 mole % of TFE, 10 to 20 mole % of HFP and 25 to 40 mole % of Et and having a melting point of about 140 to 170xc2x0 C. This patent publication does not describe the use of a peroxycarbonate as a polymerization initiator, or the formation of terminal carbonate groups as polymer chain terminal groups.
JP-B-52-24072 discloses the suspension polymerization of TFE, HFP and Et, and describes a monomeric composition of 20 to 80 mole % of TFE, 2 to 30 mole % of HFP and 20 to 60 mole % of Et. However, this patent publication describes no properties of the polymer other than a melting point.
Recently, the exhaustion of fossil energy sources such as petroleum and coal is one of crucial issues. Furthermore, environmental disruption such as the greenhouse phenomenon of the earth caused by carbon dioxide generated on combustion of petroleum or coal is an important problem in either advanced countries or developing countries. Under such circumstances, solar power generation systems have been put to practical use as alternative energy sources, which use the inexhaustible solar radiation energy.
However, the solar power generation systems are not widely spread since the production cost of solar batteries is high. Therefore, it is necessary to increase the photoelectric conversion efficiency of a whole solar cell module or to improve production and processing processes of the solar cell module, in addition to the improvement of the photoelectric conversion efficiency of photovoltaic elements such as crystalline silicon, polycrystalline silicon, amorphous silicon, copper-indium selenide, compound semiconductors, etc.
In particular, the properties of coating materials for photovoltaic elements, for example, transparency, have an intimate relationship with the increase of the photoelectric conversion efficiency of a whole solar cell module or the improvement of the production and processing processes of the solar cell module.
FIG. 3 shows a schematic cross section of a basic structure of a solar cell module. In this module, the photovoltaic element 3 is provided on the insulating substrate 4, and the transparent filler layer 2 as a protective layer of the photovoltaic element 3, and also the surface film 1 as an outermost layer (exposed on the surface of the module) are provided over the photovoltaic element 3. Accordingly, the surface film should have good light transmission, and also good weather resistance and heat resistance, since it is exposed to the sun light in the outdoors for a long time. Furthermore, the surface film is required to have impact resistance so that it can protect the photovoltaic element from external impact, anti-adhesion properties against foreign materials which deteriorate the transparency of the film, and stain-proofing properties so that such foreign materials are easily removed. In addition, when the surface film is adhered to a transparent filler layer or directly to a photovoltaic element, it should have adhesion properties with the filler layer or the photovoltaic element.
In view of such requirements, JP-A-7-297439 discloses a solar cell module which uses a surface film 1 made of a fluorine-containing polymer (e.g. a tetrafluoroethylene-ethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), polyvinyl fluoride (PVF), polyvinylidene fluoride (PVDF), a tetrafluoroethylene-hexafluoropropylene copolymer (FEP), a vinylidene copolymer, etc.), and a filler layer 2 comprising an ethylene-vinyl acetate (EVA) or a butyral resin. However, the fluorine-containing polymer inherently has low adhesion properties with other materials, and as the result, the surface film 1 and the filler layer 2 slip off, or peel off.
Thus, there are proposed an invention which uses, as an adhesive, a fluorine-containing polymer to which a hydrocarbon monomer having a carboxyl group, a carboxylic acid anhydride group, an epoxy group or a hydrolyzable silyl group such as maleic anhydride or vinylmethoxysilane is grafted (e.g. JP-A-7-18035, JP-A-7-25952, JP-A-7-25954, JP-A-7-173230, JP-A-7-173446, JP-7-173447, etc.) , or an invention using, as an adhesive between polyvinyl chloride and ETFE which is treated by corona discharge, cured adhesive composition comprising an isocyanate curing agent and a fluorine-containing polymer obtained by copolymerizing a hydrocarbon monomer having a functional group such as a hydroxyalkyl vinyl ether, with tetrafluoroethylene or chlorotrifluoroethylene (e.g. JP-A-7-228848).
Such adhesive compositions, which contains the fluorine-containing polymer comprising the grafted or copolymerized hydrocarbon monomer having a functional group, have insufficient heat resistance. Thus, they decompose or form bubbles so that the adhesion strength tends to decrease, or the film or layer is peeled off or discolored, when a composite comprising a fluorine-containing resin film is processed at a high temperature in the production process of the composite.
The first object of the present invention is to provide a fluororesin which can firmly be adhered directly to other materials such as general-purpose resins, metals, glass, crystalline silicon, polycrystalline silicon, amorphous silicon, etc. while maintaining the good properties of the fluororesin such as chemical resistance, solvent resistance, non-tackiness, electrical insulation properties, stain-proofing properties and flame retardance.
The second object of the present invention is to provide a fluororesin which can be molded at a lower temperature than conventional fluororesins, and thus can be heat bonded or co-extruded with general-purpose resins having low heat resistance.
The third object of the present invention is to provide a fluororesin having a very high transparency in comparison with conventional fluororesins.
The fourth object of the present invention is to provide a surface film or a transparent filler material for a solar cell, which has good molding and processing properties, weather resistance, adhesion properties and transparency.
The fifth aspect of the present invention is to provide a laminate comprising a fluororesin layer, in which layers are firmly bonded.
Accordingly, the present invention provides a tetrafluoroethylene copolymer comprising 30 to 81 mole % of tetrafluoroethylene and a 70 to 19 mole % of at least one other monomer, and having terminal carbonate groups at polymer chain ends, which has a melt flow rate of 0.1 to 100 g/10 min. at 200xc2x0 C. under a load of 5 kg, and a melting point of 90 to 200xc2x0 C.,
a laminate comprising a layer of the above tetrafluoroethylene copolymer, and a layer of other material, and
a surface film or a transparent filler material for a solar cell, which comprises the above tetrafluoroethylene copolymer.
In one preferred embodiment, the contents of tetrafluoroethylene and at least one other monomer in the tetrafluoroethylene copolymer are 40 to 81 mole %, and 60 to 19 mole %, respectively, the melt flow rate of the copolymer is 1.0 to 100 g/10 min. at 200xc2x0 C. under a load of 5 kg, and the copolymer has a critical shear rate of 10 to 103 secxe2x88x921 at 230xc2x0 C.